1. Field of the Invention
This invention relates to shaping of glass sheets and, in particular, to a locating and positioning system to accurately position a heat softened glass sheet at a sheet shaping station.
2a. Technical Considerations
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like and to be suitable for such applications flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side or rear windows are installed. It is also important that the side or rear windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area.
During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increase the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of the glass sheet to breakage, tempering also causes the glass sheet to fracture into relatively small, smooth surfaced fragments that are less injurious than the relative large, jagged fragments that result from the breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated glass to a desired curvature and cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace. The glass sheet is one of a series of sheets and is heated to its deformation temperature and transferred into a shaping station adjacent the furnace, where the glass sheet is pressed between upper and lower molds. The upper mold is generally a vacuum mold that holds the heat softened glass sheet by suction. At about the same time, a transfer and tempering ring having an outlined shape conforming to that desired for the glass sheet slightly inboard of its perimeter moves upstream into a position below the upper vacuum mold. Release of the vacuum deposits the glass sheet onto the tempering ring which supports the peripheral portion of the glass while it conveys the glass sheet into a cooling station for rapid cooling.
In prior art apparatus, glass sheets are lifted off the surface of conveying rolls and into engagement with the upper vacuum mold either by bottom segmented land press surfaces situated between conveying rolls, vertically reciprocating lifting fingers, or directly by suction from the vacuum mold. The segmented lower mold provides a discontinuous lifting and shaping surface. The upper contours of the lower segmented mold complement the shaping surface of the upper vacuum mold. The segmented bottom press surfaces may mark the lower surface of the glass sheet or may produce a ripple in the glass due to its discontinuous lifting and pressing surface. The vacuum mold is provided either with a lower rigidly curved shaping surface, a deformable shaping surface or a smooth flat shaping surface that lifts the flat glass sheet by suction thereagainst and depends on the release of the vacuum within the mold to permit the hot glass sheet to drop by gravity onto a tempering ring to develop the shape dictated by the outline configuration of the tempering ring. A process such as the latter is generally called drop forming.
When a flat glass sheet is shaped by drop forming, the maximum depth of bend attainable depends on the glass thickness, glass temperature and distance the glass has dropped. It is difficult to control the shape of thin glass sheets, particularly those heated to high temperatures.
Drop forming, using deformable molds, and other press bending systems employing lower segmented molds provide an efficient technique for generating cylindrical, compound, variable radii and localized configurations for simple and moderately complex patterns, where no dramatic bend geometries exist, but in order to pursue more complex geometries such as J-shaped bends, reverses, twists, sharp radii and deep localized bends, full surfaced pressing action is required. Full surface top and bottom pressing allows for complex shaping without the marking that may result from shaping with partial and discontinuous pressing surfaces.
It would be advantageous to develop a system whereby heated glass sheets could be transferred directly from the furnace to a shaping station with upper and lower full face bending molds. In addition, a system that would accurately position glass sheets within the shaping station would be desirable.
2b. Patents of Interest
U.S. Pat. Nos. 4,282,026; 4,361,432; 4,437,871; and 4,437,872 to McMaster et al. each teach a drop forming apparatus wherein a hot glass sheet is engaged within a heating furnace by a stationery upper vacuum pickup positioned above the furnace conveying rolls and subsequently is deposited on a shuttling carrier mold ring. The downwardly facing surface of the vacuum pickup can be planar or curved. The pickup can reciprocate vertically to engage the glass, or auxiliary lifters can be positioned between the furnace rolls and beneath the hot glass sheet to lift the glass for engagement with the vacuum pickup. The ring moves beneath the vacuum supported glass sheet and the vacuum is terminated to drop the hot glass sheet on the ring and effect shaping. The ring subsequently shuttles from its pickup transfer station to a quench unit that rapidly cools the shaped glass. Throughout the operation, the vacuum pickup remains horizontally stationary within the furnace and the glass is transferred directly to a ring mold.
U.S. Pat. Nos. 4,227,908; 4,229,199; 4,229,200; 4,233,049; and 4,280,828 to Seymour teach shaping glass sheets by drop forming. A heat softened glass sheet exits a furnace and is positioned on a support bed below a stationery flat vacuum pickup. The pickup lifts the glass sheet and moves upward to allow a contoured shaping ring to be positioned under the sheet. Vacuum is disengaged and the sheet drops on the ring. The force generated by the impact of the glass sheet on the ring provides the bending force required to shape the sheet and conform it to the contours of the ring. The patents also teach the use of auxiliary shapers to impart additional contours in the glass sheet.
U.S. Pat. Nos. 4,221,580; 4,285,715; and 4,433,993 to Frank and No. 4,430,110 to Frank et al. teach a horizontal press bending operation wherein heated glass sheets enter a shaping station and are lifted off the run-in conveyor rolls by a slotted lower mold. The glass sheet is pressed between the slotted lower mold and a shaped upper vacuum mold. After pressing, the lower mold is retracted to a position beneath the run-in rolls. A shuttling tempering ring is positioned below the vacuum mold and the vacuum is released so that the shaped glass is deposited onto the tempering ring. The ring subsequently transfers the shaped glass to a quenching station to temper the bent glass sheet. The upper vacuum mold can reciprocate vertically but is horizontally stationary.
U.S. Pat. No. 4,297,118 to Kellar et al. teaches a shuttling deformable vacuum mold that engages a heated glass sheet within a heating furnace. While still in the furnace, the mold deposits the shaped glass sheet on a shuttling tempering ring that is positioned beneath the mold. After depositing the glass, the vacuum mold shuttles to a position outside of the furnace to cool prior to reentering the furnace to engage the next glass sheet. The tempering ring transfers the glass sheet from the furnace to a quenching station to temper the glass.
U.S. Pat. No. 4,517,001 to McMaster teaches the use of a traveling vacuum holder with a downwardly facing engaging surface to lift a heated glass sheet and transfer the sheet onto a carrier ring mold within the heating furnace wherein the heated glass sheet is bent under the force of gravity on the mold. The bent glass sheet is subsequently removed from the furnace to a quench unit to temper the bent glass.
U.S. Pat. Nos. 4,200,420 to Cathers et al. and 4,228,993 to Cathers teach a glass sheet orienting and transporting frame for use with an industrial robot. The frame includes a plurality of sheet locating arms. The frame is positioned above the sheet and the arms locate edge portions of the sheet. The frame thereafter moves the sheet and orients it in a predetermined position. The locating arm moves away from the oriented sheet which is thereafter engaged by the frame and moved away from the sheet orienting area.
U.S. Pat. Nos. 4,204,853 and 4,298,368 to Seymour teach alignment devices for positioning glass sheets on a hot gas support bed adjacent the exit end of a furnace. Rotating conveyor rolls move the glass sheet over the support bed and urge it into contact with a locating frame. The frame is contoured to conform to the curvature of a portion of the leading edge of the glass sheet. After alignment the glass sheet is shaped.
U.S. Pat. No. 4,228,886 to Moran teaches a position sensor wherein a pair of sensors, each with multiple energy sources, e.g., a light source, direct the light source at a major surface of a glass sheet positioned thereunder. A portion of the light directed at the sheet does not contact the sheet while another portion of the light makes contact with and is reflected from the sheet. The reflected light passes through a lens and strikes a photo detector. Based on the amount of light that strikes the detector, the portion of the sheet reflecting the light from each sensor can be calculated and the overall orientation of the sheet can be determined.
U.S. Pat. No. 4,364,766 to Nitschke teaches a microprocessor based control system for monitoring and controlling pairs of hot glass sheets as they are conveyed through a heating, bending and tempering operation. The glass is conveyed through a heating furnace and as the glass pairs approach an overhead vacuum pickup in the furnace, a photo electric sensor provides a glass sensing signal to a control computer that controls the rotational velocity of different sets of furnace conveyor rolls. By controlling the roll speeds, the distance between adjacent sheets of glass pairs can be established for engagement with the vacuum pickup.
U.S. Pat. No. 4,360,374 to Nitschke teaches a glass sheet alignment system whereby the roller conveyor in the vicinity of a vacuum holder includes a roll shifter to shift sets of conveying rolls along the direction of the glass sheet conveyance. The set of rolls is shifted in the direction opposite to the direction of conveyance to reduce sliding of the glass sheet with respect to the vacuum holder as the glass sheet is engaged by the holder. In another embodiment, the conveying rolls near the vacuum holder are driven by a drive separate from the remaining rolls of the conveyor and independently controlled, to position the glass. Both arrangements align the glass sheet beneath the vacuum holder to reduce the relative movement of the hot glass sheet immediately prior to its engagement with the vacuum holder.